I am trying to write a plugin that extends the category selection box.

There will be a master-category selection box that is displayed in front of the normal category box.

I would add it to bug_report_page as follows:

<?php event_signal( 'EVENT_CATEGORY_SELECTBOX', array( $f_category_id ) ) ?>
<select <?php echo helper_get_tab_index() ?> name="category_id">
<?php
print_category_option_list( $f_category_id );
?>
</select>

Similar in bug_update_advanced_page and bug_actiongroup_page.